Like many faith-based films, I Still Believe—based on the true story of Christian rocker Jeremy Camp—has its fair share of tear-jerking scenes meant to test and enforce the spirituality of its good-hearted hero. But, because I Still Believe is also a music biopic, it’s got to make time for a few rock star moments too. After the movie’s emotional climax, Camp (Riverdale’s KJ Apa) takes the stage at a massive outdoor venue to sing the film’s titular song to a crowd of adoring fans. For the scene, sibling directors Andrew and Jon Erwin knew a green screen and thousands of CGI audience members wouldn’t cut it, so they decided to stage the real thing. At the recent I Still Believe press junket, we asked the Erwin Brothers how they pulled it off, and found out how one band’s Riverdale fandom helped bring it all together.